Adaptive optics (AO) is a revolutionary method of imaging that has been used extensively in astronomy. In such an application, for example, the blurriness of the Earth's atmosphere may be removed from starlight leading to incredible increases in resolution and sensitivity of astronomical objects. These systems have been operational at astronomical observatories now for roughly twenty years. However, these systems are currently limited by their ability to accurately measure the distortion imparted to the starlight by the atmosphere. An improved ability to sense these errors would lead to a commensurate improvement in final image sharpness. Greater sharpness leads directly to a larger discovery space for science targets.
The typical wavefront sensor for most AO systems is the Shack-Hartmann Wavefront Sensor (S-HWFS). This sensor works by dicing the light coming from a telescope into little pieces, measuring the tip/tilt aberration on each piece with a few pixels for each little piece. These few pixels are called quad-cells. When all these measurements are made, they're all stitched back together to give the full measurement. This measurement of the wavefront error is then applied to the deformable mirror in the system for correction. This sensor has been the workhorse of AO systems for more than two decades.
However, in those twenty or so years, scientific discoveries and telescope architectures have changed, thereby revealing the shortcomings of this sensor. The S-HWFS is susceptible to noise for very low order aberrations (like tip/tilt/focus). This is because for these low-order modes, many of the quad-cells measure the same signal. The additional measurements from the independent quad-cells don't improve the signal but rather add noise. Also, newer telescope architectures use segmented apertures, akin to tiles in a mosaic. Creating a large telescope from individual tiles is less risky than building a single large mirror. However, a large, tiled mirror can only behave as a single, monolithic mirror if the individual segments are properly aligned.
Another wavefront sensor that has been developed, and that can address both of these concerns is the Zernike wavefront sensor (Z-WFS). The classic Z-WFS works by taking light that is at the very center of a star image, delaying or advancing it a little bit (this is done statically, with a phase plate), and then re-imaging the light at the telescope primary mirror (or pupil) location. The classic Z-WFS has been demonstrated to be far less susceptible to noise compared to the S-HWFS for the loworder modes like tip/tilt and focus. Also, because it works using the principle of optical coherence, it can also be used to measure the relative alignment of tiled (or segmented) apertures. Unfortunately, it has a limited working dynamic range. That is, it works best when the atmospheric correction is good enough to form a sharp image of a star.
Accordingly, a need exists to develop a wavefront sensor that overcomes the fundamental deficiencies of these conventional systems.